1. Field of the Invention
The present invention relates to a solid ultrasonic delay line and, more particularly, to a solid ultrasonic delay line which has good characteristics, is easy and economical to manufacture, and has a quite high center frequency, say of the order of 100 MHz.
2. Related Art Statement
Solid ultrasonic delay lines for low-frequency signals are known, as described, for example, in Japanese Patent Publication Nos. 27574/1972 and 22818/1971 and U.S. Pat. No. 3,581,247. As shown in FIG. 7, an ultrasonic delay line of this kind has polygonal multiple-reflective surfaces including reflective surfaces 30 and 31. An input transducer 32 and an output transducer 33 are disposed on the reflective surfaces 30 and 31, respectively. Wave-absorbing material is adhered to main boundary surfaces 34 and 35 perpendicular to the reflective surfaces, or diffuse reflection surfaces 36 and 37 are formed on the boundary surfaces 34 and 35, in order to absorb or reflect spurious waves arising from transverse bulk waves which are generally used.
In recent years, high-definition television, for example, has required solid ultrasonic delay lines having quite high center frequencies, say 100 MHz. In order to satisfy this requirement, a delay line as shown in FIG. 8 has been used. Specifically, this delay line has a delay medium M of a thickness t. This thickness is set larger than the thickness of an input transducer 32 and an output transducer 33 so as not to create the possibility that waves 38 having extended side lobes are reflected to produce spurious waves.
In these ultrasonic delay lines shown FIGS. 7 and 8, a portion 39 of the delay medium M is thicker than the input transducer 32 and the output transducer 33. This portion 39 tends to produce spurious waves. Waves incident on the main boundary surfaces of the delay medium make a large incidence angle to the surfaces. Therefore, even when the wave-absorbing material is adhered to them as shown in FIG. 7, the waves are not sufficiently absorbed, but rather they tend to be directly reflected. Further, the delay medium is thick. Delay lines of this structure are fabricated one by one and hence they are expensive to manufacture.